Both physiological mapping and anatomical studies have demonstrated major differences in somatic sensory and motor cortex of mammals. These differences are reflected in the spatial separateness of the sensory and motor representations the enlarged or diminished representation of each part of the body, and the degree of development of association cortex. In rats, there are even differences within SI; SI and MI are amalgamated in the hindlimb representation, but they are separated in the face region. We propose to study cortical organization in the rat using a combination of physiological and pathway tracing techniques at light and electron microscopic levels. The rat will be used as an experimental model to explore several factors that are important to maintaining separate sensory and motor areas in rodents, before testing explicit hypotheses in primates. We will compare the corticocortical circuitry of identified face and hindlimb SI cortex to determine whether the separate SI face area has more extensive interconnections than the hindlimb cortex. Each area thus identified as interconnected with SI will be studied in a similar way to determine its connections with the thalamus and with other areas of cortex. In other studies, laminar lesions of face area SI cortex will be made in newborn animals in an attempt to establish an SI-MI amalgan for the face region that is similar to the naturally occurring amalgam of hindlimb SI cortex. The anatomical reorganization expected is that the VB projections can be induced to overlap with the VL projections in face area as they do normally in hindlimb cortex. These studies will clarify the cellular mechanisms underlying the differentiation of neocortex into more specialized subdivisions.